This research is directed toward elucidation of the enzymatic mechanisms involved in the assimilatory reduction of nitrate to ammonia. The significance of this pathway is manifest in that the various oxidized inorganic forms of nitrogen provide the ultimate nitrogen source for all life. Nitrate assimilation is a two-step process: the two-electron reduction of nitrate of nitrite, followed by the six-electron reduction of nitrite to ammonia. The enzymes catalyzing these reactions in Neurospora crassa, namely, nitrate reductase and nitrite reductase, are adaptively formed in the presence of either NO minus 3 or No minus 2. Their synthesis is repressed by NH plus 4. Nitrate reductase is a soluble, sulfhydryl-containing molybdoflavohemoprotein, mediating the NADPH-dependent reduction of nitrate via the electron transfer sequence: NADPH yields ((-SH) yields FAD yields cytochrome b-557 yields MO)) yields NO minus 3. The nitrite thus formed is stoichiometrically reduced to ammonia by nitrite reductase, also a soluble, sulfhydryl-containing flavoprotein, possessing non-heme centers and the novel iron tetrahydroporphyrin, siroheme, as prosthetic groups. The suggested electron transfer sequence is: 3NAD (P) H yields ((-SH), FAD, (Fe.S) yields siroheme) yields NO minus 2. Preliminary objectives at this point include the unambiguous elucidation of the electron transfer catalysis mediated by nitrite reductase through: a) UV/visible spectrophotometric and EPR spectroscopic analysis of the enzyme; b) kinetic studies of the overall NAD (P) H-nitrite reductase activity as well as associated partial electron-transferring functions and c) chemical and physical analyses of the nitrite reductase protein and its complement of prosthetic groups. Integration of these results should provide for reliable conclusions regarding the important relationships between structure and function in this multi-electron transferring homodimeric enzyme.